I have found that I get cross-talk between foreign languages. For example, I studied Russian in college, but I have been studying Chinese for the past several years, and now when I try to think of a Russian word, only Chinese ones come to mind. So, I am reluctant to go for a third language.
I like Chinese, because it is very different than English. I think by studying it I learn a lot about the nature of language, and get exposed to a different culture, that is very interesting.
Also, I'm getting old enough that I don't think I will have a chance to become completely fluent in third language. So, I will keep my focus on Mandarin.